The present invention relates to a powder magnetic core and a method for manufacturing same. The powder magnetic core is suitable for a transformer and a reactor for a switching power source.
Various electronic devices have been decreased in size and weight in recent years, and accordingly, a demand has increased for a miniaturization of switching power sources which are installed on electronic devices. In particular, there is a strong need for size and thickness reductions in switching power sources for use in laptop personal computers, small portable devices, thin CRT monitors, and flat panel displays. However, in the conventional switching power sources, magnetic components such as transformers and reactors, which are the main structural components thereof, take a large space, thereby limiting the reductions of size and thickness. Thus, the switching power sources are difficult to be reduced in size and thickness unless these magnetic components are made small and thin.
Metal magnetic materials such as Sendust and Permalloy or oxide magnetic materials such as ferrites have been used for magnetic components of transformers and reactors used in such switching power sources. Among them, the metal magnetic materials generally have a high saturation magnetic flux density and a magnetic permeability, but because an electric resistivity thereof is low, an eddy current loss becomes high, in particular, in a high-frequency region. Recently, a trend has been emerging towards a miniaturization of magnetic components by driving power circuits at a high frequency and decreasing a necessary inductance value, but because of the effect of eddy current loss, metal magnetic materials cannot be used at a high frequency.
On the other hand, because the oxide magnetic materials have an electric resistivity higher than that of the metal magnetic materials, the eddy current loss generated even in a high-frequency region is small. However, because the saturation magnetic flux density is small, such materials are easily magnetically saturated, thereby making it impossible to reduce their volumes. In other words, in any case, the magnetic core volume is the most significant factor determining the inductance value, and the size and thickness reductions are difficult to be attained unless the magnetic properties of magnetic materials are improved.
Thus, the possibilities for miniaturizing the conventional magnetic components are limited, and the requirements for the size and thickness reductions of electronic devices have not been fully met.
As a method for resolving these problems, a high-density sintered magnetic body has been suggested (see, for example, Japanese Unexamined Patent Application Publication No. 56-38402) in which a surface of a metal magnetic material composed of particles with a size of 1 to 10 μm is coated with a metal oxide magnetic material of a spinel composition represented by M-FexO4 (where M=Ni, Mn, Zn, x≦2).
Further, for example, International Patent Application Publication No. 03/015109 and US Patent Application Publication No. 2004/0238796 A1 suggest a composite magnetic material in which a ferromagnetic fine particulate powder of a metal or an intermetallic compound having a layer of a ferrite layer formed by plating ultrasonically excited ferrite on a surface thereof is compression molded, and a magnetic circuit is formed between the ferromagnetic particles via the ferrite layer.
Further, soft magnetic particles have been suggested, and the soft magnetic particles are composed of soft magnetic metal particles, a high-resistance substance coated on a surface thereof, and a phosphate-based conversion layer formed on a surface of the high-resistance substance, so as to obtain a soft magnetic molded body with a high density and a high specific resistance (see, for example, Japanese Unexamined Patent Application Publication No. 2001-85211).
A magnetic material has recently been suggested in which a layer of a nonmagnetic insulating oxide with a high electric resistivity is formed on the surface of soft magnetic particles with a high saturation magnetic flux density and a magnetic permeability in order to increase a resistivity and resolve a drawback of metal magnetic materials. With such a magnetic material, because the electric resistivity is increased by an effect of a nonmagnetic insulating film, it is possible to inhibit eddy current, that is, it enables the use of the magnetic material at a high frequency, e.g. in a megahertz band.
In order to further decrease the eddy current loss in a megahertz band in a soft magnetic molded body obtained by molding the above-described particles (magnetic material), it is necessary to increase the resistivity of the soft magnetic molded body by increasing the thickness of the insulating layer or high-resistance layer formed on the surface of metal particles. For example, a specific resistance in the example illustrated by Table 1 of Japanese Unexamined Patent Application Publication No. 2001-85211 is higher than that in the comparative example, but it is still insufficient. Only a material with a volume iron loss of 10 kHz is shown. To enable the operation at 1 MHz, the specific resistance of the molded body has to be raised by further increasing the thickness of the high-resistance layer. However, where the thickness of the insulating layer or high-resistance layer formed on the surface of metal particles is increased, a gap between the metal particles becomes large, and a magnetic permeability decreases. Further, where the insulating layer is made thinner to increase the magnetic permeability or the heat treatment temperature of the soft magnetic molded body obtained by press molding is raised, the decrease in resistivity causes an increase in the eddy current loss in a megahertz band.
According to another method for further decreasing the eddy current loss within a megahertz band, the thickness of a press molded powder magnetic core is decreased, and they are laminated via insulating layers (see, for example, Japanese Unexamined Patent Application Publication No. 11-74140).
Further, methods for manufacturing a soft magnetic multilayer film have also been suggested in which a laminate of soft magnetic films and insulating films is formed by alternately laminating the soft magnetic films and insulating films (see, for example, Japanese Unexamined Patent Application Publication Nos. 2000-54083 and 9-74016).
With the method disclosed in Japanese Unexamined Patent Application Publication No. 11-74140, two rings with a thickness of 5.5 mm are laminated by hot pressing so as to obtain a thickness of 10 mm. However, in thin electronic components, the total thickness is as small as 0.6 mm or less, and the thickness of the laminated body is equal to or less than half of the total thickness (for example, 0.2 mm or less). To manufacture such a thin core by press molding is also difficult from the standpoint of a mechanical strength. The degree of difficulty becomes especially significant as the surface area of the core increases. Further, because the total thickness is small, when a method of laminating thin cores via insulating layers is used, the thickness of insulating layers has to be, for example, 0.05 μm or less, but such thin sheet-like cores are substantially difficult to produce by press molding.
Japanese Unexamined Patent Applications Publications No. 2000-54083 and No. 9-74016 describe laminated structures of magnetic films and insulating films which are suitable for magnetic cores of inductors and transformers, but because the magnetic films and insulating films in both patent applications are formed by sputtering or vapor deposition, the problem is that the film formation speed is low, a significant time is required to form the laminated structure, and the thick sheet structure such as a bulk core cannot be formed due to stresses.
It is an object of the present invention to resolve the above-described problems and to provide a method for manufacturing a structure in which thin cores and insulators are alternately laminated as a method for improving high-frequency characteristics of a powder magnetic core and decreasing the eddy current loss.
Further objects and advantages of the invention will be apparent from the following description of the invention.